The present application claims priority on European Patent Application 01200177.2, filed on Jan. 18, 2001.
The present invention relates to determining the in situ effective mobility (xcex) of a formation layer. The effective mobility of a formation is defined as xcex=k/xcexc, wherein k is the formation permeability (unit Darcy, dimension L2) and wherein xcexc is the dynamic viscosity (unit Poise, dimension MLxe2x88x921Txe2x88x921). The unit of mobility xcex is Darcy/Poise and its dimension is Mxe2x88x921L3T. The formation layer is a hydrocarbon-bearing formation layer. In the specification and in the claims, the term xe2x80x98effective mobilityxe2x80x99 is used to refer to the mobility of the formation with respect to the uncontaminated formation fluid, and the term xe2x80x98mobilityxe2x80x99 is used to refer to the mobility of the formation with respect to contaminated formation fluid.
A method of determining the mobility is described in the book Wireline Formation Testing and Sampling, Schlumberger, 1996 on pages 6-3 to 6-8. The known method comprises the steps of:
a) selecting a location in the formation layer;
b) lowering in the borehole to the location a tool that comprises a central conduit having an inlet and being provided with a pressure sensor, a fluid receptacle having an inlet opening into the central conduit, and means for discharging fluid from the central conduit;
c) making an exclusive fluid communication between the formation and the inlet of the central conduit by extending into the formation a probe having an outlet that is in direct fluid communication with the inlet of the central conduit;
d) allowing formation fluid to enter into the fluid receptacle and measuring the pressure build-up; and
e) determining the effective mobility from the pressure build-up.
The mobility is determined in two stages. At first the pressure build-up curve is compared with curves determined for different regimes of fluid flow through the formation into the probe. This comparison allows selecting an actual flow regime. Then the mobility is calculated from the measured data and the selected actual flow regime.
It will be understood that when the dynamic viscosity is known, the formation permeability can be calculated from the mobility.
This is called a pre-test build-up analysis. A disadvantage of the pre-test build-up analysis is that one determines the mobility of the formation with respect to the drilling mud that invaded the formation during drilling. Because the formation fluid is contaminated, its viscosity will not be the same as the viscosity of the uncontaminated formation fluid, and thus this pretest mobility will not be the same as the mobility of the formation with respect to the formation hydrocarbons.
However, Applicant had found that the pre-test build-up analysis can be used to determine an average value of the true or effective formation permeability.
To this end, the method of determining the average in situ permeability of a formation layer traversed by a borehole according to the present invention comprises the steps of
a) selecting a set of locations in the formation layer;
b) selecting from the set a first location;
c) lowering in the borehole to the location a tool that comprises a central conduit having an inlet and being provided with a pressure sensor, a fluid receptacle having an inlet opening into the central conduit, a fluid analyser, and means for discharging fluid;
d) making an exclusive fluid communication between the formation and the inlet of the central conduit;
e) allowing formation fluid to pass through the central conduit, allowing the formation fluid to enter into the fluid receptacle, and measuring the pressure build-up;
f) determining the mobility from the pressure build-up;
g) positioning the tool near a next location and repeating steps d) through f) until the mobilities of the locations in the set have been determined;
h) determining for one location of the set the effective mobility, calculating the permeability for this location using the known viscosity of the uncontaminated formation fluid, and determining the viscosity of contaminated formation fluid using the permeability and the mobility determined in step f) for that location; and
k) calculating the permeabilities for the other locations of the set using the viscosity of the contaminated formation fluid and the mobility determined in step f), and calculating the average of the permeabilities,
wherein determining the effective mobility, which is the mobility of the formation with respect to the uncontaminated formation fluid, comprises the steps of
1) selecting a location in the formation layer;
2) lowering in the borehole to the location a tool that comprises a central conduit having an inlet and being provided with a pressure sensor, a fluid receptacle having an inlet opening into the central conduit, a fluid analyser, and means for discharging fluid;
3) making an exclusive fluid communication between the formation and the inlet of the central conduit;
4) allowing formation fluid to pass through the central conduit, analysing the fluid, allowing the formation fluid to enter into the fluid receptacle when the fluid is the substantially uncontaminated formation fluid, and measuring the pressure build-up; and
5) determining the effective mobility from the pressure build-up.
It will be understood that it takes some time before the drilling mud is displaced and uncontaminated formation fluid enters into the central conduit. However, this is not such a large drawback, because, in general, a sample of the uncontaminated formation fluid is also needed, so that the pressure build-up test according to the invention can be carried out after a sample has been taken.